Over the years, many advances have been made in the process of converting wood pulp into paper. The papermaking art includes depositing a layer of fibrous pulp, or stock, onto a moving screen and draining excessive water from the fibrous stock to form a relatively thin, fibrous web on the surface of the screen. In order to dry the web as well as increase its strength, the web is removed from the screen and passed through various pressing rolls to reduce the amount of water remaining in the web. After passage through the press section of a papermaking machine, where the web's density has been increased, the web is conducted around multiple heated drums, or dryers, such that excess water remaining in the web is removed.
In a modern papermaking machines, it is not uncommon to have in the drying section eighty or more dryer drums, with each drum having a diameter of five feet and a length of up to 33 feet. Of necessity, these dryer drums take up an enormous floor area which greatly exceeds the floor area required for the installation of the forming and pressing section combined. Additionally, with escalating fuel costs, it is evident that if more water can be removed in the press section, less energy will be required in the drying section. Consequently, much research has been carried out in an attempt to remove ever greater quantities of water from the paper web before it leaves the press section. This reduces the number of dryer drums required and the amount of energy and steam required to drive off any remaining moisture within the web.
A breakthrough in press section design was commercialized in 1980 by the introduction of the so-called "extended nip press" (herein referred to as an ENP), which successfully increases the percent fiber in the web after pressing from 35 present to approximately fifty percent. The main features of the ENP, as compared to the prior conventional pressing technique, may be outlined as follows.
In conventional pressing, the web passes through the nip defined by counter-rotating rolls, whereas in the ENP, one of the rolls is replaced by a concave shoe. The concave surface of the shoe cooperates with the outer surface of a press roll to define therebetween an elongated or extended pressing section, such that the web is pressed with a moderate pressure for an extended period of time between the press roll and the shoe. In order to permit the web to pass through the extended nip, a moveable blanket is disposed between the concave surface and the web, so that the web is pressed between the blanket and the pressing roll during passage through the press section.
More recent improvements in the ENP have involved solving a problem of oil leakage from the nip blanket by extending the blanket beyond the ends of the backing roll where the ends may be made to take a circular shape and sealing them to rotating heads.
Further improvements in the function of the ENP have been achieved by induction heating the surface of the backing, or press roll, in the temperature range of 200 to 600 degrees Fahrenheit. The addition of heat to the ENP to form a so-called impulse dryer has resulted not only in better drying of the web, but also increased mechanical properties, such as web strength. Increased web strength can have tremendous pay-back in reducing the cost of paper production.
The ability to manufacture increased strength paper or paperboard can allow changes in the furnish, the fibers used to construct the paper. The ability to develop greater strength in a paper or paperboard web allows the use of lower-cost, and lower-strength fibers, or the use of a lower-weight paper for a particular application. Using less fiber, or fiber of lower cost, can yield a dramatic cost savings, particularly for heavy-weight paper or paperboard materials.
What is needed is a combination press dryer which can provide improved drying and strength development, particularly for heavier weight paper webs.